Sterilization system employing a switching module adapted to pulsate the low frequency power applied to a plasma

ABSTRACT

A sterilization system and method applies low frequency power to a plasma within a vacuum chamber to remove gas or vapor species from an article. The sterilization system includes a switching module adapted to pulsate the low frequency power applied to the plasma and a low frequency power feedback control system for controllably adjusting the low frequency power applied to the plasma. A power monitor is adapted to produce a first signal indicative of the low frequency power applied to the plasma within the vacuum chamber. A power control module is adapted to produce a second signal in response to the first signal from the power monitor, and a power controller is adapted to adjust, in response to the second signal, the low frequency power applied to the plasma to maintain a substantially stable average low frequency power applied to the plasma while the article is being processed.

CLAIM OF PRIORITY

[0001] This application is a continuation-in-part of and claims priorityfrom U.S. Utility patent application Ser. No. 09/677,534, filed Oct. 2,2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to systems and methods for controlling gasdischarge plasmas in sterilization systems that employ gas dischargeplasmas.

[0004] 2. Description of the Related Art

[0005] Plasmas produced using radio frequency (RF) generators inparticular have proven to be valuable tools in processes for thesterilization of medical devices. For example, in U.S. Pat. Nos.4,643,876 and 4,756,882, which are incorporated by reference herein,Jacobs, et al. disclose using hydrogen peroxide as a precursor in a lowtemperature sterilization system that employs RF plasma. The combinationof hydrogen peroxide vapor and a RF plasma provides an efficient methodof sterilizing medical devices without using or leaving highly toxicmaterials or forming toxic by-products. Similarly, Jacob, U.S. Pat. No.5,302,343, and Griffiths, et al., U.S. Pat. No. 5,512,244, teach the useof RF plasmas in a sterilization process.

[0006] However, there are problems associated with the use of an RFplasma in a sterilization process. The RF plasma may leave residualhydrogen peroxide on the sterilized article. The residual amount ofhydrogen peroxide remaining on the sterilized article depends upon theRF power applied to the article, the amount of time exposed to the RFplasma, and the material of the article. For example, while someplastics (e.g., polyurethane) absorb hydrogen peroxide, other materials(e.g., Teflon) absorb relatively little, thereby yielding less residualhydrogen peroxide after sterilization.

[0007] In addition, inherent inefficiencies in the energy conversionfrom the low frequency (e.g., 60 Hz) line voltage to the RF (e.g.,approximately 1 MHz - 1 GHz) voltage used to generate the RF plasmalimit the power efficiency of such systems to typically less than 50%.Energy efficiency is further reduced by typically 5 -20% by virtue ofthe losses from the required impedance matching network between the RFgenerator and the load. Such low energy efficiency significantlyincreases the cost per watt applied to the sterilized articles. Therequired instrumentation for using RF electrical energy (e.g., RFgenerator, impedance matching network, monitoring circuitry) isexpensive, which also increases the cost per watt applied to thesterilized articles.

SUMMARY OF THE INVENTION

[0008] One aspect of the present invention is a sterilization systemthat applies low frequency power to a plasma within a vacuum chamber toremove gas or vapor species from an article. The low frequency power hasa frequency less than or equal to approximately 200 kHz. Thesterilization system comprises a switching module adapted to pulsate thelow frequency power applied to the plasma. The sterilization systemfurther comprises a low frequency power feedback control system forcontrollably adjusting the low frequency power applied to the plasma.The low frequency power feedback control system comprises a powermonitor adapted to produce a first signal indicative of the lowfrequency power applied to the plasma within the vacuum chamber. The lowfrequency power feedback control system further comprises a powercontrol module adapted to produce a second signal in response to thefirst signal from the power monitor, and a power controller adapted toadjust, in response to the second signal, the low frequency powerapplied to the plasma to maintain a substantially stable average lowfrequency power applied to the plasma while the article is beingprocessed.

[0009] Another aspect of the present invention is a method ofcontrollably adjusting a low frequency power applied to a plasma withina vacuum chamber of a sterilization system to remove gas or vaporspecies from an article. The low frequency power has a frequency lessthan or equal to approximately 200 kHz. The method comprises pulsatingthe low frequency power applied to the plasma and monitoring the lowfrequency power applied to the plasma within the vacuum chamber. Themethod further comprises generating a first signal indicative of the lowfrequency power applied to the plasma. The method further comprisesadjusting the low frequency power applied to the plasma in response tothe first signal to maintain a substantially stable average lowfrequency power applied to the plasma while the article is beingprocessed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 schematically illustrates a preferred embodiment of asterilization system compatible with the present invention.

[0011]FIG. 2A schematically illustrates a preferred embodiment of acylindrically-shaped electrode with open ends and perforated sides.

[0012]FIG. 2B schematically illustrates an alternative embodiment of acylindrically-shaped electrode with open ends and louvered sides.

[0013]FIG. 2C schematically illustrates an alternative embodiment of acylindrically-shaped electrode with open ends and solid sides.

[0014]FIG. 2D schematically illustrates an alternative embodiment of anelectrode comprising one or more colinear cylindrically-shaped segmentswith open ends and solid sides.

[0015]FIG. 2E schematically illustrates an alternative embodiment of anelectrode with a partial cylindrical shape, open ends, and solid sides.

[0016]FIG. 2F schematically illustrates an alternative embodiment of acylindrically-symmetric and longitudinally-asymmetric electrode withopen ends and solid sides.

[0017]FIG. 2G schematically illustrates an alternative embodiment of oneor more asymmetric electrodes with open ends and solid sides.

[0018]FIG. 2H schematically illustrates an alternative embodiment of anelectrode system with a first electrode that is cylindrically-shapedwith open ends and solid sides, and a second electrode comprising a wiresubstantially colinear with the first electrode.

[0019]FIG. 2I schematically illustrates an alternative embodiment of agenerally square or rectangular electrode within a generally square orrectangular vacuum chamber.

[0020]FIG. 3, which is broken into FIGS. 3A and 3B, schematicallyillustrates an embodiment of a low frequency power module compatiblewith the phase angle control method of the present invention.

[0021]FIG. 4, which is broken into FIGS. 4A and 4B, schematicallyillustrates an embodiment of a low frequency power module compatiblewith the amplitude control method of the present invention.

[0022]FIG. 5A schematically illustrates the phase angle control methodof controlling the low frequency power applied to the plasma.

[0023]FIG. 5B schematically illustrates the amplitude control method ofcontrolling the low frequency power applied to the plasma.

[0024]FIG. 6, which is broken into FIGS. 6A and 6B, schematicallyillustrates an embodiment of a low frequency power module comprising aswitching module compatible with the present invention.

[0025]FIG. 7 schematically illustrates the output from a switchingmodule compatible with the present invention which pulsates an incomingsinusoidal voltage with a frequency of 60 Hz to form a series of voltagepulses with widths W and spaced by times T.

[0026]FIG. 8A schematically illustrates a rippled DC voltage generatedby full-wave-rectifying and filtering the line voltage prior topulsating the voltage via the switching module.

[0027]FIG. 8B schematically illustrates the rippled DC voltage of FIG.8A after being pulsated by the switching module utilizing unipolarswitching.

[0028]FIG. 8C schematically illustrates the rippled DC voltage of FIG.8A after being pulsated by the switching module utilizing bipolarswitching.

[0029]FIG. 9 schematically illustrates one embodiment of a switchingmodule which generates a unipolar pulsation.

[0030]FIG. 10 schematically illustrates a preferred embodiment of amethod of sterilization compatible with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0031] Production of gas discharge plasmas using low frequency (LF)voltages avoids the various problems inherent in the state of the artsterilization devices and processes which form and use plasmas producedby radio frequency (RF) voltages. First, LF plasma processing leavesless residual reactive species remaining on the sterilized articles thandoes RF plasma processing. Second, generation of the LF plasma is highlyenergy efficient because little or no frequency conversion from the linevoltage is needed. For example, by using no frequency conversion with aline voltage frequency of 60 Hz, the energy efficiency of thesterilization system can reach approximately 85-99%. Use of LF voltagesalso does not require an impedance matching network, thereby avoidingthe associated energy losses. Third, due to the simplifiedinstrumentation and higher energy efficiency of LF generation, the costper watt applied to the sterilized articles using LF plasmas can be aslow as one-tenth the cost per watt of using RF plasmas. Fourth, thesimplified instrumentation used for generating LF plasmas has proven tobe more reliable and robust, and requiring less complicated diagnosticinstrumentation.

[0032]FIG. 1 schematically illustrates one preferred embodiment of thepresent invention comprising a sterilization system 10. Thesterilization system 10 comprises a vacuum chamber 12, a vacuum pump 14,a vacuum pump line 15, a vacuum pump valve 16, a reactive agent source18, a reactive agent line 19, a reactive agent valve 20, a low frequency(LF) power module 22, an LF voltage conduit 24, a vent 26, a vent line27, a vent valve 28, a process control module 30, an electrode 32, and areactive agent monitor 34. Persons skilled in the art recognize thatother embodiments comprising sterilization systems of differentconfigurations than that illustrated in FIG. 1 are compatible with thepresent invention.

[0033] In the preferred embodiment of the present invention, thearticles (not shown in FIG. 1) to be sterilized are packaged in variouscommonly employed packaging materials used for sterilized products. Thepreferred materials are spunbonded polyethylene packaging materialcommonly available under the trademark “TYVEK” or composites of “TYVEK”with a polyethylene terephthalate packaging material commonly availableunder the trademark “MYLAR”. Other similar packaging materials may alsobe employed such as polypropylene. Paper packaging materials may also beused. With paper packaging, longer processing times may be required toachieve sterilization because of possible interactions of the reactiveagent with paper.

[0034] The vacuum chamber 12 of the preferred embodiment is sufficientlygas-tight to support a vacuum of approximately less than 40 Pa (0.3Torr). Coupled to the vacuum chamber 12 is a pressure monitor (notshown) which is also coupled to the process control module to provide ameasure of the total pressure within the vacuum chamber. Also coupled tothe vacuum chamber 12 is the reactive agent monitor 34 which is capableof detecting the amount of the reactive agent in the vacuum chamber 12.In the exemplary embodiment of the present invention, the reactive agentis hydrogen peroxide, and the reactive agent monitor 34 measures theabsorption of ultraviolet radiation at a wavelength characteristic ofhydrogen peroxide. Other methods of reactive agent detection compatiblewith the present invention include, but are not limited to, pressuremeasurement, near infrared absorption, and dew point measurement. Thereactive agent monitor 34 is also coupled to the process control module30 to communicate the detected amount of the reactive agent to theprocess control module 30.

[0035] In the preferred embodiment of the present invention, inside andelectrically isolated from the vacuum chamber 12 is the electrode 32,which is electrically conductive and perforated to enhance fluidcommunication between the gas and plasma species on each side of theelectrode 32. The electrode 32 of the preferred embodiment generallyconforms to the inner surface of the vacuum chamber 12, spacedapproximately one-half to two inches from the wall of the vacuum chamber12, thereby defining a gap region between the vacuum chamber 12 and theelectrode 32. The electrode 32 is coupled to the LF power module 22 viathe LF voltage conduit 24. In the preferred embodiment, with the vacuumchamber 12 connected to electrical ground, application of an LF voltagebetween the vacuum chamber 12 and the electrode 32 creates an LFelectric field which is stronger in a first region 31 which includes thegap region and the vicinity of the edges of the electrode 32. The LFelectric field is weaker in a second region 33 where the sterilizedarticles are placed. Generally, in other embodiments, the LF electricfield can be generated by applying an LF voltage between the electrode32 and a second electrode in the vacuum chamber 12. In such embodiments,the first region 31 includes the gap region between the two electrodes,and the vicinity of the edges of one or both of the electrodes. Thepreferred embodiment in which the vacuum chamber 12 serves as the secondelectrode is one of the many different ways to generate the gas plasma.

[0036] In the preferred embodiment illustrated in FIG. 2A, acylindrically-shaped electrode 32 provides fluid communication betweenthe gas and plasma on each side of the electrode 32 through the openends of the electrode 32 as well as through the perforations in the sideof the electrode 32. These open ends and perforations permit gaseous andplasma species to freely travel between the first region 31 between theelectrode 32 and the walls of the vacuum chamber 12 and the secondregion 33 where the sterilized articles are placed. Similarly, asillustrated in FIGS. 2B-2I, other configurations of the electrode 32provide fluid communication between the first region 31 and the secondregion 33. FIG. 2B schematically illustrates a cylindrically-shapedelectrode 32 with open ends and louvered openings along its sides. FIG.2C schematically illustrates a cylindrically-shaped electrode 32 withopen ends and solid sides. FIG. 2D schematically illustrates anelectrode 32 comprising a series of colinear cylindrically-shapedsegments with open ends and solid sides. FIG. 2E schematicallyillustrates an electrode 32 with a partial cylindrical shape, open endsand solid sides. FIG. 2F schematically illustrates acylindrically-symmetric and longitudinally-asymmetric electrode 32 withopen ends and solid sides. FIG. 2G schematically illustrates anasymmetric electrode 32 with open ends and solid sides. More than oneelectrode can be used to generate the plasma. FIG. 2H schematicallyillustrates an electrode system with a first electrode 32 that iscylindrically-shaped with open ends and solid sides, and a secondelectrode 32′ comprising a wire substantially colinear with the firstelectrode 32. The LF voltage is applied between the first electrode 32and the second electrode 32′. In this embodiment, the first region 31 isthe region between the first electrode 32 and the second electrode 32′,and the second region 33 is between the first electrode 32 and thevacuum chamber 12. FIG. 21 schematically illustrates a generally squareor rectangular electrode within a generally square or rectangular vacuumchamber. The various configurations for generally cylindrical electrodesschematically illustrated in FIGS. 2A-2H can also be applied to thegenerally square or rectangular electrode of FIG. 2I. Each of theseembodiments of the electrode 32 provide fluid communication between thefirst region 31 and the second region 33.

[0037] The vacuum pump 14 of the preferred embodiment is coupled to thevacuum chamber 12 via the vacuum pump line 15 and the vacuum valve 16.Both the vacuum pump 14 and the vacuum pump valve 16 are coupled to, andcontrolled by, the process control module 30. By opening the vacuumvalve 16, gases within the vacuum chamber 12 are pumped out of thevacuum chamber 12 through the vacuum pump line 15 by the vacuum pump 14.In certain embodiments, the vacuum valve 16 is capable of being openedto variable degrees to adjust and control the pressure in the vacuumchamber 12.

[0038] The reactive agent source 18 of the preferred embodiment is asource of fluid coupled to the vacuum chamber 12 via the reactive agentline 19 and the reactive agent valve 20. The reactive agent valve 20 iscoupled to, and controlled by, the process control module 30. Thereactive agent source 18 of the preferred embodiment comprises reactiveagent species. In the preferred embodiment, the reactive agent speciescomprises a germicide which is a sterilant or a disinfectant, such ashydrogen peroxide. In addition, the germicide supplied by the reactiveagent source 18 can be in gas or vapor form. By opening the reactiveagent valve 20, reactive agent atoms and molecules from the reactiveagent source 18 can be transported into the vacuum chamber 12 via thereactive agent line 19. In certain embodiments, the reactive agent valve20 is capable of being opened to variable degrees to adjust the pressureof the reactive agent in the vacuum chamber 12. In the exemplaryembodiment of the present invention, the reactive agent species of thereactive agent source 18 comprising hydrogen peroxide molecules.

[0039] The vent 26 of the preferred embodiment is coupled to the vacuumchamber 12 via the vent line 27 and the vent valve 28. The vent valve 28is coupled to, and controlled by, the process control module 30. Byopening the vent valve 28, vent gas is vented into the vacuum chamber 12via the vent line 27. In certain embodiments, the vent valve 28 iscapable of being opened to variable degrees to adjust the pressure ofthe air in the vacuum chamber 12. In the exemplary embodiment of thepresent invention, the vent 26 is a High Efficiency Particulate-filteredAir (HEPA) vent which provides filtered air as the vent gas. Other ventgases compatible with the present invention include, but are not limitedto, dry nitrogen, and argon.

[0040] The process control module 30 is coupled to various components ofthe sterilization system 10 to control the sterilization system 10. Inan exemplary embodiment of the present invention, the process controlmodule 30 is a microprocessor configured to provide control signals tothe various other components in response to the various signals receivedfrom other components.

[0041] The LF power module 22 of the preferred embodiment is coupled tothe electrode 32 via the LF voltage conduit 24, and is coupled to, andcontrolled by, the process control module 30. The LF power module 22 isadapted to apply a low frequency voltage between the electrode 32 andthe vacuum chamber 12 so as to generate a plasma in the vacuum chamber12. FIG. 3, which is broken into FIGS. 3a and 3 b, schematicallyillustrates an embodiment of the LF power module 22 compatible with thephase angle control method of controlling the low frequency powerapplied to the plasma. As illustrated in FIG. 3, the LF power module 22comprises an over-power relay 40, a pair of metal oxide varistors 42, astep-up transformer 50, a flyback current shunt element 62, an inductor64, a capacitor 66, and a LF power feedback control system 70. The LFpower feedback control system 70 illustrated in FIG. 3 comprises a powercontroller 60, a current monitor 80, a voltage monitor 90, and a powermonitor 100 coupled to the current monitor 80 and the voltage monitor90. Line voltage (typically 200-240 VAC, 50/60 Hz) is provided to thestep-up transformer 50 via the closed over-power relay 40 which iscoupled to the LF power feedback control system 70.

[0042] In the embodiment illustrated in FIG. 3, the metal oxidevaristors (MOVs) 42 are used to suppress transient voltage impulses.Each MOV 42 is a multiple-junction solid-state device capable ofwithstanding large magnitude impulses with a low amount of let-throughvoltage. The MOVs 42 serve as fast acting “variable resistors” with alow impedance at higher-than-normal voltages and a high impedance atnormal voltages. MOVs are manufactured for specific voltageconfigurations and for a variety of impulse magnitudes. Persons skilledin the art are able to select MOVs 42 consistent with the presentinvention.

[0043] The output voltage of the step-up transformer 50 is preferablybetween approximately 100 and 1000 V_(rms), more preferably betweenapproximately 200 and 500 V_(rms), and most preferably betweenapproximately 250 and 450 V_(rms). The output voltage of the step-uptransformer 50 is transmitted to the power controller 60, which providesthe LF voltage to the electrode 32 and vacuum chamber 12 via the flybackcurrent shunt element 62, the inductor 64, the capacitor 66, and the LFpower feedback control system 70. The flyback current shunt element 62provides a path for fly-back current and to tune the circuit, and in thepreferred embodiment the flyback current shunt element 62 is a loadresistor of approximately 1500 ohms. In other embodiments, the flybackcurrent shunt element 62 can be a snubber. The inductance of theinductor 64 is chosen to limit noise spikes in the LF current, and istypically approximately 500 mH. The capacitance of the capacitor 66 ischosen to maximize the efficiency of power transfer to the LF plasma bymatching the resonant frequency of the series LC circuit to thefrequency of the applied LF voltage. For a 60 Hz voltage and aninductance of 500 mH, a capacitance of approximately 13.6 μF providesthe resonant condition for which the impedance of the series LC circuitis approximately zero, thereby maximizing the transmitted LF power.Persons skilled in the art are able to select appropriate values forthese components depending on the frequency of the applied LF voltage ina manner compatible with the present invention.

[0044]FIG. 4, which is broken into FIGS. 4a and 4 b, schematicallyillustrates an embodiment of the LF power module 22 compatible with theamplitude control method of controlling the low frequency power appliedto the plasma. As illustrated in FIG. 4, the LF power module 22comprises an over-power relay 40, a pair of metal oxide varistors 42, astep-up transformer 55, and a LF power feedback control system 70. TheLF power feedback control system 70 illustrated in FIG. 4 comprises ahigh voltage (HV) DC power supply 51, a voltage-controlled oscillator(VCO) 52, a voltage-controlled amplifier (VCA) 53, a HV operationalamplifier 54, a current monitor 80, a voltage monitor 90, and a powermonitor 100 coupled to the current monitor 80 and the voltage monitor90. Line voltage is provided to the HV DC power supply 51 via the closedover-power relay 40 which is coupled to the LF power feedback controlsystem 70. The output of the HV DC power supply 51 is preferably betweenapproximately 100 and 1000 VDC, more preferably between approximately200 and 500 VDC, and most preferably between approximately 250 and 450VDC.

[0045] In the embodiment illustrated in FIG. 4, the VCO 52 generates asinewave output with a constant amplitude and fixed low frequency lessthan or equal to approximately 200 kHz (which includes DC voltages andcurrents), the low frequency selected by supplying an appropriateset-point voltage to the VCO 52. Alternative embodiments can utilizeother waveforms, e.g., triangular or square waveforms. The LF output ofthe VCO 52 is supplied to the VCA 53, which serves as a power controllerto maintain a substantially stable average LF power applied to theplasma. In response to a feedback signal from the power control module110, the VCA 53 amplifies the LF output of the VCO 52 to generate anamplified LF voltage with an amplitude between approximately 0 and 12VAC. The amplified LF voltage from the VCA 53 is supplied to the HVoperational amplifier 54 which in response generates a high voltage LFoutput with an amplitude determined by the amplitude of the amplified LFvoltage from the VCA 53. Appropriate HV operational amplifiers arecommercially available (e.g., Apex Microtechnology, Tuscon, Ariz., partnumber PA93), and persons skilled in the art are able to select a HVoperational amplifier compatible with the present invention. Typically,the amplitude of the high voltage LF output from the HV operationalamplifier 54 is approximately 100 to 150 VAC. In order to generatelarger amplitude LF voltages to be applied to the plasma, the highvoltage LF output from the HV operational amplifier 54 can be furtheramplified by the step-up transformer 55, as illustrated in FIG. 4.Alternatively, the step-up transformer 55 may be omitted if the HVoperational amplifier 54 is capable of generating a high voltage LFoutput with the desired amplitude to be applied to the plasma.

[0046] In both the phase angle control embodiment illustrated in FIG. 3and the amplitude control embodiment illustrated in FIG. 4, the LF powerfeedback control system 70 of the LF power module 22 further comprises apower control module 110 coupled to the power monitor 100, which iscoupled to the current monitor 80 and voltage monitor 90. The currentmonitor 80 measures the LF current through the electrode 32 and thevacuum chamber 12. In the preferred embodiment of the present invention,the current monitor 80 includes a current sensor 82 which provides avoltage output indicative of the measured real-time, cycle-by-cycle LFcurrent, a first converter 84 which produces a DC voltage in response tothe RMS of the voltage output of the current sensor 82, and a firstvoltage amplifier 86 which amplifies the DC voltage from the firstconverter 84 to produce a real-time current signal. In addition, thecurrent monitor 80 also includes an over-current detector 88, whichmonitors the DC voltage from the first converter 84 in real-time andsends an error signal to the power control module 110 if the LF currentexceeds a pre-set value, caused for example by a plasma instabilitybetween the electrode 32 and the vacuum chamber 12. Under such anoccurrence, the LF voltage is turned off momentarily. This occurrencecan result in a few cycles being lost, however the LF power isstabilized so that the average power is not affected by more than apredetermined tolerance.

[0047] The voltage monitor 90 measures the LF voltage between theelectrode 32 and the vacuum chamber 12. In the preferred embodiment ofthe present invention, the voltage monitor 90 includes a step-downtransformer 92 which produces a voltage output indicative of themeasured real-time, cycle-by-cycle LF voltage, a second converter 94which produces a DC voltage in response to the RMS of the voltage outputof the step-down transformer 92, and a second voltage amplifier 96 whichamplifies the DC voltage from the second converter 94 to produce areal-time voltage signal. In other embodiments, the voltage output fromthe step-down transformer 92 and the voltage output of the currentsensor 82 are multiplied together and directed to a single converterwhich creates a voltage signal corresponding to the power applied to theplasma.

[0048] In the preferred embodiment, the power monitor 100 furthercomprises a multiplier that receives the DC voltages from the currentmonitor 80 and the voltage monitor 90, and multiplies these two voltagesto produce a real-time power signal proportional to the LF power appliedto the plasma between the electrode 32 and the vacuum chamber 12, thereal-time power signal being generated in response to the real-timecurrent and real-time voltage signals, and transmitted to the powercontrol module 110. In other embodiments, the power monitor 100 monitorsthe LF power applied to the plasma by utilizing a signal indicative ofthe real-time impedance of the plasma with either the real-time currentor real-time voltage signals. In still other embodiments, the powermonitor 100 monitors the LF power applied to the plasma by utilizingother real-time signals which indirectly indicate the LF power appliedto the plasma; e.g., a real-time signal proportional to the brightnessof the glow discharge generated by the plasma. Persons skilled in theart can select an appropriate power monitor 100 compatible with thepresent invention.

[0049] The power control module 110 of the preferred embodiment includesa fault detector, such as an over-power detector 112 which monitors thereal-time power signal from the power monitor 100 and opens theover-power relay 40 if the LF power exceeds a pre-set value, therebyextinguishing the LF plasma. After such an occurrence, the control ofrestart can be given to the user or to software. The power controlmodule 110 of the preferred embodiment further comprises an additionalfault detector, such as a thermal switch 114 which detects overheating,and a power control processor 120.

[0050] In the preferred embodiment, the power control processor 120controls and monitors the status of the LF power feedback control system70. The power control processor 120 is coupled to a user interface 122which provides user input regarding a selected power magnitude settingand a selected power on/off setting. The power control processor 120 isalso coupled to the power monitor 100, the thermal switch 114, and theover-current detector 88. In the preferred embodiment, the powermagnitude setting can be selected from two power levels. When the LFpower is turned on, the preferred embodiment of the power controlprocessor 120 ensures that a “soft start” condition is maintained inwhich the inrush current is minimized. In addition, the user interface122 receives signals from the power control processor 120 indicative ofthe status of the sterilization system 10, which is communicated to theuser.

[0051] In the phase angle control embodiment illustrated in FIG. 3, thepower control processor 120 is also coupled to the power controller 60.In this embodiment, the power control processor 120 transmits a signalto the power controller 60 in response to signals from the userinterface 122, power monitor 100, over-current detector 88, and thermalswitch 114 in order to maintain a substantially stable LF power appliedto the LF plasma while avoiding error conditions. In the amplitudecontrol embodiment illustrated in FIG. 4, the power control processor120 is coupled to the VCA 53. In this embodiment, the power controlprocessor 120 transmits a signal to the VCA 53 in response to signalsfrom the user interface 122, power monitor 100, over-current detector88, and thermal switch 114 in order to maintain a substantially stableLF power applied to the LF plasma while avoiding error conditions. Inboth embodiments illustrated in FIG. 3 and FIG. 4, the power controlprocessor 120 typically maintains the LF power applied to the LF plasmawithin a tolerance of approximately 0- 5% of the specified power level.

[0052] Note that not all of the components listed and described in FIG.3 and FIG. 4 are required to practice the present invention, since FIG.3 and FIG. 4 merely illustrate particular embodiments of the LF powermodule 22. These components include components for automation, safety,regulatory, efficiency, and convenience purposes. Other embodimentscompatible with the present invention can eliminate some or all of thesecomponents, or can include additional components.

[0053] In response to the signal from the power control processor 120,the power controller 60 of the embodiment illustrated in FIG. 3 controlsthe LF power applied between the electrode 32 and the vacuum chamber 12by utilizing phase angle control. Under phase angle control, the dutycycle of the LF power is modified by zeroing the voltage and currentapplied between the electrode 32 and the vacuum chamber 12 for a portionΔ of the cycle period. Such phase angle control is often used tomaintain constant power from electric heaters or furnaces. FIG. 5Aschematically illustrates the voltage and current for a 100% duty cycle(i.e., Δ=0) and for a reduced duty cycle (i.e., Δ≠0). During normaloperations, the power controller 60 maintains a constant LF powerapplied to the plasma by actively adjusting the duty cycle of the LFpower in response to the feedback real-time signal received from thepower control module 110 in response to the measured LF power. When afault event is detected by the over-current detector 88 or thermalswitch 114, the power control processor 120 reduces the LF power byreducing the duty cycle of the LF power, and it transmits a signal tothe user interface 122 to provide notification of the fault event.Persons skilled in the art are able to select appropriate circuitry tomodify the duty cycle of the LF power consistent with the presentinvention.

[0054] Alternatively, the LF power can be controlled by utilizingamplitude control, as in the embodiment illustrated in FIG. 4. Underamplitude control, the LF power is modified by adjusting the amplitudeof the voltage and current applied between the electrode 32 and thevacuum chamber 12. FIG. 5B schematically illustrates the voltage andcurrent corresponding to a first LF power setting and a second LF powersetting less than the first LF power setting. During normal operations,the VCA 53 maintains a constant LF power applied to the plasma byactively adjusting the amplitude of the LF power in response to thefeedback real-time signal received from the power control module 110 inresponse to the measured LF power. Persons skilled in the art are ableto select appropriate circuitry to modify the amplitude of the LF powerconsistent with the present invention.

[0055] The electronics for RF sterilizers are complicated by the need ofsuch systems to attempt to closely match the output impedance of the RFgenerator with the plasma impedance at all times in order to maximizepower efficiency and to avoid damage to the RF generator. Plasmaimpedance varies widely during plasma formation, being very high untilthe plasma is fully formed, and very low thereafter. When first ignitinga plasma, the RF generator cannot match the high plasma impedance whichexists prior to the full formation of the plasma, so a large fraction ofthe power output is reflected back to the RF generator. RF generatorshave protection systems which typically limit the RF generator outputduring periods of high reflected power to avoid damage. However, toignite the plasma, the voltage output of the RF generator must exceedthe threshold voltage required for plasma ignition. The thresholdvoltage is dependent on the chamber pressure, reactive agent, and otheroperating geometries and parameters and is approximately 300 V_(rms) inthe preferred embodiment. In an RF system, once ignition has beenachieved, and the plasma impedance is thereby reduced, the magnitude ofthe applied RF voltage must be reduced to a sustaining voltage, e.g.,approximately 140 V_(rms), to avoid excessive power delivery. Becausethe higher RF voltages required for plasma ignition produce excessivelyhigh reflected power before full plasma formation, RF generators requirecomplicated safeguards to prevent damage during the plasma ignitionstage.

[0056] Conversely, the complexity of the power system and rate ofignition failures are significantly reduced for LF sterilizers since theLF sterilizers may operate using applied voltages above the thresholdvoltage and have much less restrictive output impedance matchingrequirements. For frequencies below approximately 1 kHz, during thetimes at which the applied LF voltage equals zero, as seen in FIG. 5A,the LF plasma is extinguished and there is no LF plasma in the vacuumchamber. The LF plasma must then be re-ignited twice each cycle. By onlyoperating in one voltage regime, LF sterilizers have simpler and morereliable electrical systems than do RF sterilizers. These electricalsystems are easier to service and diagnose, thereby reducing the costsassociated with repair. In addition, the higher peak plasma densitiesresulting from LF sterilizers likely result in increased dissociativerecombination on the articles, thereby reducing the amount of residualreactive species remaining on the articles after the sterilizationprocedure.

[0057] In certain embodiments, localized gas heating causes reductionsof the plasma impedance, which then generate arcing or other plasmainstabilities. This condition is more likely to occur where currentdensities are larger. However, this effect is reduced, and stability isincreased, for frequencies higher than the 50/60 Hz of the incoming linevoltage. As schematically illustrated in FIG. 6, which is broken intoFIGS. 6a and 6 b, certain embodiments of the LF power module 22 may alsoinclude a switching module 130 to provide higher frequencies bypulsating the LF power applied to the plasma. While FIG. 6 is based onthe embodiment illustrated in FIG. 3, a switching module 130 can also beused by making a similar modification to the embodiment of FIG. 4. WhileFIG. 6 shows the switching module 130 inserted between the first MOV 42and the step-up transformer 50, persons skilled in the art appreciatethat other embodiments can have the switching module 130 at otherlocations within the LF power module 22.

[0058]FIG. 7 schematically illustrates the output from a switchingmodule 130 compatible with the present invention. As shown in FIG. 7, anincoming sinusoidal voltage with a frequency of 60 Hz is pulsated by theswitching module 130 to form a series of voltage pulses with widths Wand spaced by times T. Persons skilled in the art appreciate that otherwaveforms may also be inputted into the switching module 130. Forexample, in order to minimize or eliminate periods in which the appliedvoltage is less than the threshold voltage V_(T) required for plasmaignition, a rippled DC voltage can be generated by full-wave-rectifyingand filtering the line voltage prior to pulsating the voltage via theswitching module 130, as illustrated in FIG. 8a. In one embodiment, sucha rippled DC voltage can be generated by inserting a AC-to-DC converterin the device schematically illustrated by FIG. 6 between the over powerrelay 40 and the MOV 42. Persons skilled in the art can identify otherembodiments which can generate such a rippled DC voltage.

[0059] For illustrative purposes, the output of the switching module 130illustrated in FIG. 7 has a frequency of approximately 1000 Hz. Personsskilled in the art appreciate that other frequencies are compatible withthe present invention. The output frequency of the switching module ispreferably less than or equal to approximately 200 kHz (which includesDC voltages and currents), more preferably between approximately 1 kHzto 100 kHz, and most preferably between approximately 20 kHz to 50 kHz.In addition, the switching module 130 can be configured to provide anoutput with unipolar pulsation, as illustrated in FIG. 8b, or withbipolar pulsation, as illustrated in FIG. 8c. Bipolar pulsation reducesthe possibility of electrode sputtering by the plasma and can provide amore stable plasma. FIG. 9 schematically illustrates one embodiment of aswitching module 130 which generates a unipolar pulsation by utilizing abridge rectifier 131, a power MOSFET 132, a MOSFET driver 133, a snubber134, a opto-isolator 135, and a function generator 136. The snubber 134is used to shunt the energy stored in the leakage inductance of thestep-up transformer 50 so it does not damage the power MOSFET 132. Anexample of an opto-isolator 135 compatible with the present invention isthe 4N26 phototransistor optocoupler, sold by Texas Instruments ofDallas, Tex. An example of a power MOSFET 132 compatible with thepresent invention is the “Super-247,” serial number IRFPS37N50A, sold byInternational Rectifier of El Segundo, Calif. An example of a MOSFETdriver 133 compatible with the present invention is the SN75372 dualMOSFET driver, sold by Texas Instruments of Dallas, Tex.

[0060] In certain embodiments, such as that illustrated in FIG. 9, theLF power applied to the plasma can also be adjusted by the switchingmodule 130 by adjusting pulse widths W or times T between pulses. Incertain embodiments, the function generator 136 can be replaced by apulse width modulation (PWM) controller that automatically controls theapplied LF power. While the period (W+T) is the inverse of the selectedpulsating frequency, the ratio of W/T can be varied to control theaverage LF power applied to the plasma. In such embodiments, theswitching module 130 can receive appropriate signals from the powercontrol processor 120 to appropriately select the pulse widths W and/ortimes T.

[0061]FIG. 10 schematically illustrates a preferred method ofsterilization using the apparatus schematically illustrated in FIG. 1.The sterilization process shown in FIG. 10 is exemplary, and personsskilled in the art recognize that other processes are also compatiblewith the present invention. The preferred process begins by sealing 200the article to be sterilized into the vacuum chamber 12. The vacuumchamber is then evacuated 210 by engaging the vacuum pump 14 and thevacuum valve 16 under the control of the process control module 30. Thevacuum chamber 12 is preferably evacuated to a pressure of less thanapproximately 1320 Pa (10 Torr), more preferably between approximately25 to 270 Pa (0.2 to 2 Torr), and most preferably between approximately40 to 200 Pa (0.3 to 1.5 Torr).

[0062] In an exemplary process, upon reaching a desired pressure in thevacuum chamber 12, the process control module 30 signals the LF powermodule 22 to energize the electrode 32 within the vacuum chamber 12. Byapplying a LF voltage to the electrode 32, the LF power module 22ionizes the residual gases in the vacuum chamber 12, thereby creating220 a gas discharge LF plasma inside the vacuum chamber 12. This gasdischarge LF plasma is formed from the residual gases in the vacuumchamber 12, which are primarily air and water vapor. Because this gasdischarge LF plasma is created 220 before the reactive agent is injectedinto the vacuum chamber 12, this gas discharge LF plasma is typicallycalled the “pre-injection” plasma. The vacuum valve 14 is controllablyopened and closed to maintain a preset vacuum pressure during thepre-injection plasma step 220. The pre-injection plasma heats thesurfaces inside the vacuum chamber 12, including the articles, therebyaiding the evaporation and removal of condensed water and other absorbedgases from the vacuum chamber 12 and the articles. A similarpre-injection plasma is described by Spencer, et al. in U.S. Pat. Nos.5,656,238 and 6,060,019, which are incorporated by reference herein. Inan exemplary process, the pre-injection plasma is turned off afterapproximately 0 to 60 minutes. Other embodiments that are compatiblewith the present invention do not include the creation of thepre-injection plasma, or use multiple pre-injection plasmas. In stillother embodiments, the vacuum chamber 12 can be vented after thearticles are exposed to the pre-injection plasma.

[0063] In the preferred process, upon reaching a desired chamberpressure, the vacuum valve 16 is closed, and the reactive agent valve 20is opened under the control of the process control module 30, therebyinjecting 230 reactive agent from the reactive agent source 18 into thevacuum chamber 12 via the reactive agent line 19. In the preferredembodiment, the reactive agent comprises hydrogen peroxide, which isinjected in the form of a liquid which is then vaporized. The injectedliquid contains preferably from about 3% to 60% by weight of hydrogenperoxide, more preferably from about 20% to 60% by weight of hydrogenperoxide, and most preferably from about 30% to 60% by weight ofhydrogen peroxide. The concentration of hydrogen peroxide vapor in thevacuum chamber 12 may range from 0.125 to 20 mg of hydrogen peroxide perliter of chamber volume. The higher concentrations of hydrogen peroxidewill result in shorter sterilization times. Air or inert gas such asargon, helium, nitrogen, neon, or xenon may be added to the chamber withthe hydrogen peroxide to maintain the pressure in the vacuum chamber 12at the desired level. This injection 230 of reactive agent may occur asone or more separate injections.

[0064] Due to this injection 230 of reactive agent, the chamber pressureof the preferred process rises to approximately 2000 Pa (15 Torr) ormore. After approximately 6 minutes into the injection stage 230, thereactive agent is permitted to diffuse 240 completely and evenlythroughout the vacuum chamber 12. After approximately 1-45 minutes ofdiffusing 240, the reactive agent is substantially in equilibrium insidethe vacuum chamber 12. This diffusing 240 allows the reactive species todiffuse through the packaging material of the articles, and come intoclose proximity, if not contact, with the surfaces of the articles,thereby sterilizing the articles. In other embodiments, the diffusion ofthe reactive agent can be immediately followed by a vent of the vacuumchamber 12.

[0065] The vacuum chamber 12 is then partially evacuated 250 by pumpingout a fraction of the reactive agent from the vacuum chamber 12 bycontrollably opening the vacuum valve 16 under the control of theprocess control module 30. Once the vacuum pressure within the vacuumchamber 12 has reached the desired pressure, the vacuum valve 16 iscontrollably adjusted to maintain the desired pressure, and the processcontrol module 30 signals the LF power module 22 to energize theelectrode 32 within the vacuum chamber 12. In the preferred embodimentin which the reactive agent comprises hydrogen peroxide, the pressure ofthe hydrogen peroxide in the vacuum chamber 12 is preferably less thanapproximately 1320 Pa (10 Torr), more preferably between approximately25 and 270 Pa (0.2 to 2 Torr), and most preferably between approximately40 and 200 Pa (0.3 to 1.5 Torr). By applying a LF voltage to theelectrode 32, the LF power module 22 generates 260 a reactive agent LFplasma inside the vacuum chamber 12 by ionizing the reactive agent. Thearticle is exposed to the reactive agent LF plasma for a controlledperiod of time. In the preferred embodiment, an additional cycle 275 isperformed. Other embodiments may omit this additional cycle 275, or mayinclude further cycles.

[0066] In both RF and LF plasmas, the components of the reactive agentplasma include dissociation species of the reactive agent and moleculesof the reactive agent in excited electronic or vibrational states. Forexample, where the reactive agent comprises hydrogen peroxide as in thepreferred embodiment, the reactive agent plasma likely includes chargedparticles such as electrons, ions, various free radicals (e.g., OH,O₂H), and neutral particles such as ground state H₂O₂ molecules andexcited H₂O₂ molecules. Along with the ultraviolet radiation produced inthe reactive agent plasma, these reactive agent species have thepotential to kill spores and other microorganisms.

[0067] Once created, the charged particles of the reactive agent plasmaare accelerated by the electric fields created in the vacuum chamber 12.Because of the fluid communication between the first region 31 and thesecond region 33, some fraction of the charged particles created in thefirst region 31 are accelerated to pass from the first region 31 to thesecond region 33 which contains the articles.

[0068] Charged particles passing from the first region 31 to the secondregion 33 have their trajectories and energies affected by the electricpotential differential of the sheath regions between the plasma and thewalls of the vacuum chamber 12 and the electrode 32. These sheathregions are created by all electron-ion plasmas in contact with materialwalls, due to charged particles impinging from the plasma onto thewalls. Electrons, with their smaller mass and hence greater mobility,are lost from the plasma to the wall before the much heavier and lessmobile ions, thereby creating an excessive negative charge densitysurrounding the walls and a corresponding voltage differential whichequalizes the loss rates of the electrons and the ions. This voltagedifferential, or sheath voltage, accelerates electrons away from thewall surface, and accelerates positive ions toward the wall surface.

[0069] The sheath voltage varies for different plasma types,compositions, and methods of production. For RF plasmas, the sheathvoltage is typically 40%-80% of the RF voltage applied to the electrode32. For example, for a root-mean-squared (RMS) RF voltage of 140 V_(rms)applied to the electrode 32 once the RF plasma is established, thecorresponding sheath voltage is approximately 55-110 V. An ion enteringthe sheath region surrounding the electrode 32 will then be acceleratedto an energy of 55-110 eV. This acceleration of positive ions by thesheath voltage is the basic principle behind semiconductor processing byRF plasmas.

[0070] As described above, for the LF plasmas of the preferredembodiment of the present invention, the voltage applied to theelectrode 32 may be equal to or greater than the ignition thresholdvoltage, which is typically 300 V. In addition, for LF plasmas, thesheath voltage is typically a higher percentage of the applied voltagethan for RF plasmas, so the sheath voltage of the preferred embodimentof the present invention is then much higher than the sheath voltage foran RF plasma system. This higher sheath voltage thereby accelerates thecharged particles of the LF plasma to much higher energies. Therefore,because the charged particles are accelerated to higher energies, thecharged particles of the LF plasma of the preferred embodiment travelfarther and interact more with the articles than do the chargedparticles of RF plasma sterilizers.

[0071] Since the LF electric field changes polarity twice each cycle,the direction of the electric field acceleration on the chargedparticles reverses twice each cycle. For charged particles in the firstregion 31, this oscillation of the direction of the acceleration resultsin an oscillation of the position of the charged particles. However,because of the fluid communication between the first region 31 and thesecond region 33, some fraction of the charged particles are able topass to the second region 33 containing the articles from the firstregion 31 before the direction of the electric field accelerationreverses.

[0072] The fraction of the charged particles created in the reactiveagent LF plasma which enter the second region 33 is a function of thefrequency of the applied electric field. The charged particles have twocomponents to their motion—random thermal speed and drift motion due tothe applied electric field. The thermal speed, measured by thetemperature, is the larger of the two (typically approximately 10⁷-10⁸cm/sec for electrons), but it does not cause the charged particles toflow in any particular direction. Conversely, the drift speed isdirected along the electric field, resulting in bulk flow of chargedparticles in or opposed to the direction of the applied electric field.The magnitude of the drift speed is approximately proportional to themagnitude of the applied electric field, and inversely proportional tothe mass of the charged particle. In addition, the magnitude of thedrift speed is dependent on the gas species and chamber pressure. Forexample, for typical operating parameters of gas discharge plasmasterilizers, including an average electric field magnitude ofapproximately 1 volt/cm, the drift speed for an electron formed in a gasdischarge plasma is typically approximately 10⁶ cm/sec.

[0073] A charged particle enters the second region 33 containing thearticles only if it reaches the second region 33 before the polarity ofthe applied electric field changes, which would reverse the accelerationof the charged particle away from the electrode 32. For example, for anapplied RF electric field with a frequency of 13.56 MHz, the period ofthe electric field is approximately 7.4×10⁻⁸ sec, so an electron onlymoves a distance of approximately 3.7×10⁻³ cm during the half-cycle orhalf-period before the direction of the electric field changes and theelectron is accelerated away from the electrode 32. Due to their muchlarger masses, ions move much less than do electrons. Where the firstregion 31 between the vacuum chamber 12 and the electrode 32 isapproximately 2.54 cm wide, as in the preferred embodiment, only afraction of the charged particles created by an RF plasma would actuallyreach the second region 33 containing the articles.

[0074] Conversely, for an applied LF electric field with a frequency of60 Hz, the period of the electric field is approximately 16.7×10⁻³ sec,so an electron can move approximately 8.35×10³ cm before it isaccelerated away from the electrode 32. Therefore, the use of LFvoltages to create the plasma in the sterilization system 10 of thepreferred embodiment results in more activity in the second region 33,as compared to a plasma generated using RF voltages. This higheractivity in LF sterilizers likely contributes to the increasedefficiency for the removal of residual reactive species from thesterilized articles as compared to RF sterilizers.

[0075] The plasma decay time, defined as a characteristic time for theplasma to be neutralized after power is no longer applied, provides anapproximate demarcation between the LF and RF regimes. The plasma decaytime is not known precisely, but it is estimated to be approximately10⁻⁴-10⁻³ sec for the plasma densities used in sterilizer systems, suchas the preferred embodiment of the present invention. This plasma decaytime corresponds to the time a charged particle exists before it isneutralized by a collision with a surface or another plasma constituent,and is dependent on the plasma species generated and the geometries ofthe various components of the sterilization system 10. As describedabove, the LF regime is characterized by a plasma which is extinguishedand re-ignited twice each cycle, i.e., the half-period of the applied LFvoltage is greater than the plasma decay time. Therefore, thesterilization system 10 is continually run at an applied voltage abovethe ignition threshold voltage of the plasma in order to re-ignite theplasma. The estimated approximate range of plasma decay times of10⁻⁴-10⁻³ sec for many of the plasmas compatible with the presentinvention then translates to an upper limit on the low frequency regimeof approximately 1-10 kHz. However, under certain circumstances, higherfrequencies can be tolerated.

[0076] Alternatively, the upper limit of the low frequency regime may bedefined as the frequency at which the electron drift speed is too slowfor an electron to traverse the 2.54-cm-wide first region 31 during ahalf-period of the applied LF voltage. Under typical operatinggeometries, this upper limit of the low frequency regime would beapproximately 200 kHz. For other geometries, the upper limit of the lowfrequency regime can be correspondingly different.

[0077] In the preferred method, the LF power module 22 remains energizedfor approximately 2-15 minutes, during which the plasma removes excessresidual reactive species present on surfaces within the vacuum chamber12, including on the articles. There is a brief rise of the vacuumpressure upon generating 260 the plasma, however, the majority of theresidual removal step 270 is conducted at an approximately constantvacuum pressure of 50 to 70 Pa (0.4 to 0.5 Torr). The residual removalstep 270 is ended by the process control module 30, which turns off theLF power module 22, thereby quenching the plasma.

[0078] After the residual removal step 270, the vacuum chamber 12 isvented 280 by the process control module 30 which opens the vent valve28, thereby letting in vent gas from the vent 26 through the vent line27 and the vent valve 28. In the preferred process, the vacuum chamber12 is then evacuated 290 to a pressure of approximately 40 to 105 Pa(0.3 to 0.8 Torr) to remove any remaining reactive agent which may bepresent in the vacuum chamber 12. The vacuum chamber 12 is then ventedagain 300 to atmospheric pressure, and the sterilized articles are thenremoved 310 from the vacuum chamber 12.

[0079] The LF plasma provides a reduction of the amount of residualreactive agent molecules remaining on the articles after thesterilization procedure is complete. Where the reactive agent compriseshydrogen peroxide, the amount of residual hydrogen peroxide remaining onthe sterilized articles is preferably less than approximately 8000 ppm,more preferably less than approximately 5000 ppm, and most preferablyless than approximately 3000 ppm. In a comparison of the amount ofresidual hydrogen peroxide remaining after a LF plasma sterilization ascompared to a RF plasma sterilization, nine polyurethane test sampleswere exposed to hydrogen peroxide during a simulated sterilization cyclein both a LF sterilizer and a RF sterilizer. Each sample was prepared bywashing with Manuklenz® and drying prior to sterilization to avoid anycross contamination. The nine samples were then distributed uniformlyacross the top shelf of a standard industrial rack.

[0080] A full LF sterilization cycle, which matched nearly exactly theconditions of a standard RF sterilizer cycle, was used to perform thecomparison. The full LF sterilization cycle included a 20-minuteexposure to a pre-injection plasma, a first 6-minute hydrogen peroxideinjection, a vent to atmosphere, a 2-minute diffusion, a first 2-minutepost-injections plasma, a second 6-minute hydrogen peroxide injection, avent to atmosphere, a 2-minute diffusion, a second 2-minutepost-injection plasma, and a vent to atmosphere. Two full LFsterilization cycles were performed and compared to two full RFsterilization cycles. As seen in Table 1, all parameters other than thepost-injection plasma power were maintained as constant as possible fromrun to run. TABLE 1 LF Run 1 LF Run 2 RF Run 1 RF Run 2 Pre-injectionplasma power 727 W 779 W 751 W 752 W First post-injection plasma 783 W874 W 757 W 756 W power Second post-injection 755 W 893 W 758 W 758 Wplasma power Chamber temp. 45° C. nom. 45° C. nom. 45° C. nom. 45° C.nom. Injection system temp. 65-75° C. 65-75° C. 65-75° C. 65-75° C. H₂O₂concentration 17 mg/l 17 mg/l 17 mg/l 17 mg/l Chamber pressure during 50Pa (0.4 Torr) 50 Pa (0.4 Torr) 50 Pa (0.4 Torr) 50 Pa (0.4 Torr) plasma

[0081] Variations in the pre-plasma power were ±3.5%, so the sampletemperature was approximately constant from run to run. The samples werethen removed and the residual analysis was performed.

[0082] The LF sterilizer used to generate the LF plasma was operated at60 Hz, and with an inductor of 500 mH and a capacitor of 13.6 μF. LFplasma power was determined by multiplying the voltage across the LFplasma by the current, then averaging on an oscilloscope. Thefluctuation level of the LF power was approximately 10%. Table 2illustrates the results of the comparison. TABLE 2 LF Run 1 LF Run 2 RFRun 1 RF Run 2 Average post- 769 W 884 W 757 W 757 W injection plasmapower H₂O₂ residuals 1973 ± 144 1864 ± 75 2682 ± 317 2510 ± 203 (ppm)

[0083] Exposure to a LF post-injection plasma reduced the residualreactive species more effectively than did exposure to a RFpost-injection plasma of comparable power. LF Run 1 had approximately23% less residual hydrogen peroxide than either RF Run 1 or RF Run 2,even though all had approximately the same post-injection plasma power.

[0084] The LF processes therefore resulted in less residual hydrogenperoxide than did the corresponding RF process.

[0085] The comparison of the two LF sterilization cycles illustratesthat increased plasma power results in a reduction of the hydrogenperoxide residuals. Furthermore, the variation between samples, asindicated by the standard deviation of the residual measurements, wassignificantly reduced in the LF process, thereby indicating an increaseduniformity as compared to the RF process.

[0086] Although described above in connection with particularembodiments of the present invention, it should be understood thedescriptions of the embodiments are illustrative of the invention andare not intended to be limiting. Various modifications and applicationsmay occur to those skilled in the art without departing from the truespirit and scope of the invention as defined in the appended claims.

What is claimed is:
 1. A sterilization system that applies low frequencypower to a plasma within a vacuum chamber to remove gas or vapor speciesfrom an article, the low frequency power having a frequency less than orequal to approximately 200 kHz, the sterilization system comprising: aswitching module adapted to pulsate the low frequency power applied tothe plasma; a low frequency power feedback control system forcontrollably adjusting the low frequency power applied to the plasma,the low frequency power feedback control system comprising: a powermonitor adapted to produce a first signal indicative of the lowfrequency power applied to the plasma within the vacuum chamber; a powercontrol module adapted to produce a second signal in response to thefirst signal from the power monitor; and a power controller adapted toadjust, in response to the second signal, the low frequency powerapplied to the plasma to maintain a substantially stable average lowfrequency power applied to the plasma while the article is beingprocessed.
 2. The sterilization system as described in claim 1, whereinthe switching module utilizes unipolar switching to pulsate the lowfrequency power applied to the plasma.
 3. The sterilization system asdescribed in claim 1, wherein the switching module utilizes bipolarswitching to pulsate the low frequency power applied to the plasma. 4.The sterilization system as described in claim 1, wherein the switchingmodule is further adapted to adjust pulse widths of the low frequencypower applied to the plasma in response to signals from the powercontrol module, thereby controlling the average low frequency powerapplied to the plasma.
 5. The sterilization system as described in claim1, wherein the switching module is further adapted to adjust timesbetween pulses of the low frequency power applied to the plasma inresponse to signals from the power control module, thereby controllingthe average low frequency power applied to the plasma.
 6. Thesterilization system as described in claim 1, wherein the low frequencypower feedback control system further comprises a current monitor thatis adapted to produce a third signal indicative of a current applied tothe plasma, and a voltage monitor that is adapted to produce a fourthsignal indicative of a voltage applied across the plasma.
 7. Thesterilization system as described in claim 6, wherein the power monitoris adapted to produce the first signal in response to the third signaland the fourth signal.
 8. The sterilization system as described in claim6, wherein the current monitor comprises a current sensor, a firstconverter, and a first voltage amplifier.
 9. The sterilization system asdescribed in claim 8, wherein the current monitor further comprises anover-current detector coupled to the power control module.
 10. Thesterilization system as described in claim 6, wherein the voltagemonitor comprises a step-down transformer, a second converter, and asecond voltage amplifier.
 11. The sterilization system as described inclaim 1, wherein the power control module comprises a power controlprocessor.
 12. The sterilization system as described in claim 11,wherein the power control module further comprises a fault detector. 13.The sterilization system as described in claim 12, wherein the faultdetector is selected from the group consisting of an over-power detectorand a thermal switch.
 14. The sterilization system as described in claim11, wherein the power control processor is coupled to the powercontroller, the power monitor, and the current monitor.
 15. Thesterilization system as described in claim 1, wherein the power controlmodule is coupled to a user interface adapted to receive user input andto transmit the user input to the power control module.
 16. Thesterilization system as described in claim 1, wherein the powercontroller is adapted to adjust a duty cycle of the low frequency powerapplied to the plasma in response to the second signal from the powercontrol module.
 17. The sterilization system as described in claim 1,wherein the power controller is adapted to adjust an amplitude of thelow frequency power applied to the plasma in response to the secondsignal from the power control module.
 18. The sterilization system asdescribed in claim 1, wherein the low frequency power has a frequencyless than or equal to approximately 200 kHz.
 19. The sterilizationsystem as described in claim 1, wherein the low frequency power has afrequency from approximately 1 kHz to approximately 100 kHz.
 20. Amethod of controllably adjusting a low frequency power applied to aplasma within a vacuum chamber of a sterilization system to remove gasor vapor species from an article, the low frequency power having afrequency less than or equal to approximately 200 kHz, the methodcomprising: pulsating the low frequency power applied to the plasma;monitoring the low frequency power applied to the plasma within thevacuum chamber; generating a first signal indicative of the lowfrequency power applied to the plasma; and adjusting the low frequencypower applied to the plasma in response to the first signal to maintaina substantially stable average low frequency power applied to the plasmawhile the article is being processed.
 21. The method as described inclaim 20, wherein the pulsating of the low frequency power applied tothe plasma is unipolar.
 22. The method as described in claim 20, whereinthe pulsating of the low frequency power applied to the plasma isbipolar.
 23. The method as described in claim 20, wherein the adjustingof the low frequency power applied to the plasma comprises adjustingpulse widths of the low frequency power applied to the plasma.
 24. Themethod as described in claim 20, wherein the adjusting of the lowfrequency power applied to the plasma comprises adjusting times betweenpulses of the low frequency power applied to the plasma.
 25. The methodas described in claim 20, wherein the monitoring of the low frequencypower applied to the plasma comprises: monitoring a current applied tothe plasma and generating a second signal indicative of the current; andmonitoring a voltage applied across the plasma and generating a thirdsignal indicative of the voltage.
 26. The method as described in claim25, wherein the generating of the first signal is in response to thesecond signal and the third signal.
 27. The method as described in claim20, wherein the adjusting of the low frequency power applied to theplasma comprises adjusting a duty cycle of the low frequency powerapplied to the plasma.
 28. The method as described in claim 20, whereinthe adjusting of the low frequency power applied to the plasma comprisesadjusting an amplitude of the low frequency power applied to the plasma.29. The method as described in claim 20, wherein the low frequency powerhas a frequency less than or equal to approximately 200 kHz.
 30. Themethod as described in claim 20, wherein the low frequency power has afrequency from approximately 1 kHz to approximately 100 kHz.